The invention relates to an efficient process and device for the decontamination of waters polluted with heavy metals, semimetals and/or radionuclides, the process according to the invention being efficiently applicable also in those cases where the metals and/or radionuclides are present as cations and anions dissolved together in water. According to the invention, metals present both in cationic and anionic forms and/or radionuclides can be eliminated from these waters down to values where discharge is possible.
For example, waters polluted with heavy metals, semimetals and/or radionuclides are e.g. landfill leachates, aqueous solutions and extracts of soils, sludges, industrial refuse, and municipal waste, as well as process waters and waste waters from energy-producing, material-processing, municipal and agricultural enterprises.
As a result of the industrial utilization of mineral resources, metals have been and are being introduced into the biosphere to a considerable extent. Heavy metal pollution is particularly notable at sites where industrial metal production and metalworking are performed, e.g. near smelting works, metal-processing or electroplating factories. In addition to deposition of such emissions in subsurface soil horizons in the vicinity of such emission sources of heavy metals, they also produce substantial amounts of waste water polluted with heavy metals. Prior to being discharged into the receiving water, these waste waters must be made free of heavy metal pollution down to limiting values where discharge is possible.
However, considerable amounts of heavy metal-bearing, radioactive waste, including waste waters radioactively polluted with radionuclides, also arises in the industrial production of electricity in nuclear plants and in the production and processing of nuclear fuels. Among the various waste waters from nuclear plants used to produce electricity, those from the controlled areas represent a special sector of problems. Such contaminated waste waters may only be discharged into the receiving waters within extremely small limiting values that will do no harm to creatures in the environment, and therefore, they must be made free of radioactive contaminations as well.
Waste waters from controlled areas of nuclear power plants include fission and activation products. Most of the fission products formed in a nuclear reactor are short-lived, which is why they rarely occur—also due to diffusion—in the water of the primary circuit.
In contrast, activation products are present in the water of the primary circuit at substantially higher concentrations. Above all, these are radioisotopes (i.e., radionuclides) of the elements iron, nickel, cobalt, manganese, chromium, zinc, and antimony, which are formed by neutron activation from the alloy components of the reactor materials used.
The metals from waste waters from the metal-processing industry will not be destroyed by technical processes but rather, they will merely be displaced or can be recovered, and therefore, techniques and processes are required which would allow removal and, in some cases, even recovery of the metals from the waste waters. In case of more heavily metal-polluted process waste waters from various metal-processing industrial sectors, methods of removing the major amounts of polluting metals are well-known, e.g. precipitation, adsorption, ion exchange, electrolytic deposition, membrane processes (electrodialysis, reversed osmosis), and biosorption. A common feature in these processes is their decreasing effectiveness in case of low, yet toxic metal concentrations (especially in case of radionuclides).
To eliminate the radioisotopes, the radioactively polluted waste waters can be processed according to the same processes described above where, as a rule, only a combination of some of these different processes will result in significant decontamination complying with the regulations which are even stricter in this case. According to the present state of the art in nuclear power plants, the activity level of the waste waters to be discharged is cut down by means of filtration, chemical flocculation and precipitation reactions, ion exchange, electrodialysis, and/or reversed osmosis. Depending on each process used separately, decontamination factors of only 10 to 1000 are achieved which, however, are insufficient for discharge and require the above-described use of various decontamination techniques or dilution steps with non-polluted process water to reach harmless activities. Decontamination factors of 1,000 to 10,000 are only achieved by means of distillation (evaporation) of radioactively polluted waste waters. This extremely energy-intensive decontamination method is the one most widely used in nuclear power plants, but still requires additional separation processes or dilution steps.
In summary, it is to be stated that quite a number of methods for the decontamination of such waste waters bearing heavy metals and/or radionuclides are known and put to technical use. However, all these methods are highly complex or energy-intensive and work by themselves only at high concentrations or in combination with each other at low concentrations of metal-bearing pollutants in the waste waters. Moreover, they leave large amounts of hazardous waste difficult to dispose of.
Adsorption, biosorption and ion exchange are special and frequently used embodiments of decontaminating environmentally hazardous waste waters containing metals, including radionuclides.
For example, the U.S. Pat. Nos. 5,080,806 and 5,262,062 describe methods of adsorptive binding of heavy metals to alkali salts of humic acids under strongly alkaline conditions.
According to U.S. Pat. No. 4,616,001, active charcoals recovered from natural materials by coking and secondary treatment with superheated steam exhibit adsorptive binding properties for heavy metals and radionuclides. However, these heavy metal adsorbers have only low binding capacity.
Finding suitable bioadsorbers based on natural materials is a central issue in developing materials which bind heavy metals. Bioadsorbers are derived from inexpensive and readily accessible sources of raw materials, e.g. from agriculture and forestry, or from the microbe-related industry, and they are biologically degradable in general. Biotechnol. Prog. 1995, 11, 235–250, describes a variety of such bioadsorbers.
Attempts have been made to increase the low binding capacity of many bioadsorbers by chemical modification.
Thus, for example, ion exchangers having improved binding properties for heavy metals and radionuclides present as cations in aqueous solution are produced by phosphorylation of raw and waste materials from agriculture and forestry or from the microbe-related industry. According to DE 196 03 786 A1, e.g. ion exchangers having considerably increased binding capacity for heavy metals are produced by phosphorylation of biomass from micro-organisms of the genuses Aspergillus, Penicillium, Trichoderma or Micrococcus. Polysaccharide-containing raw materials from agriculture and forestry can also be converted to ion exchangers having high binding capacity for cationic heavy metals and radionuclides. For example, using phosphorylation with various phosphorylating agents, bioadsorbers having heavy metal-binding properties are produced from cellulose (FA-A-2,206,977), lignocellulose (WO 93/11196), wood chips (DE 42 39 749 A1), sawdust (JP 87-267663), paper pulp (JP 86-234543), and starch (JP 92-308078), which can be used in the decontamination of waste waters containing heavy metals and radionuclides therein.
It is also familiar to deposit heavy metal cations by electrolytic means. Thus, according to U.S. Pat. No. 5,587,064, heavy metal cations included in waste waters can be deposited cathodically on a cathode and scraped off mechanically with a device so as to allow repeated use of the device.
Another method of decontaminating waste waters including heavy metal cations has been set forth in the U.S. Pat. Nos. 5,019,273 and 5,092,563. The method comprises addition of e.g. aluminum turnings to the waste water. The heavy metals included in the waste water are reduced to the elements and thus precipitated. This method suffers from the drawback that an approximately 200fold excess of aluminum turnings must be used and that the process gives rise to huge salt loads in the effluent water.
A common feature in all of the processes of heavy metal and/or radionuclide decontamination described so far is their decreasing effectiveness at low, yet—especially in the radionuclide range—toxic metal concentrations. Therefore, highly concentrated metal solutions must be subjected to a pretreatment using the above-described methods so as to obtain values allowing discharge in the subsequent treatment with ion exchangers, for example.
Quite a number of heavy metals and semimetals such as manganese, chromium, molybdenum, antimony, tungsten, and arsenic are present in the form of anionic oxo compounds in aqueous solution, in most cases in the presence of cationic heavy metals and radionuclides as well. Thus, they cannot be eliminated from waste waters using cation exchangers. However, low-cost anion exchangers and methods of eliminating heavy metals present in anionic form in waste waters using anions exchangers for particularly low concentration ranges are not available as yet. Therefore, according to U.S. Pat. No. 4,222,872, heavy metals present as anions in waste waters, for example, are precipitated by means of precipitation reactions using e.g. ferric salts and filtrated off. Heavy metals present as anions in waste waters can also be precipitated by reduction and alkalinizing the waste water as set forth in U.S. Pat. No. 4,705,639. However, precipitation reactions do not result in the limiting values of metals in waste waters to be discharged, not to speak of the low values in waste waters of nuclear plants. According to the prior art, polymeric, water-insoluble anion exchangers are nevertheless used to remove fission and activation products from the waste waters in nuclear power plants. However, this gives rise to large amounts of hazardous waste difficult to dispose of. Moreover, the highly stringent values for fission and activation products in waste waters neither are achieved in this way, but only by means of additional secondary treatment processes stressing environment and resources.
With higher demands as to the purity level of the waste waters to be discharged, there is a considerable rise in cost and equipment-related input of processes for the decontamination of waste waters including heavy metals and/or radionuclides. In particular, this applies to waste waters from nuclear plants.